This invention relates to a novel design for an integrated electronic microphone, and in particular to such a microphone that can be manufactured by microfabrication techniques, and to methods of manufacturing such a microphone.
Miniaturized microphones are used in a wide range of applications, such as cell phones, hearing aids, smart toys and surveillance devices for example. It would be desirable to design such microphones and manufacturing methods therefor that allow batch production and allow the integration of the microphone with miniaturised devices having other functions. Microfabrication of various types of miniature microphones has been explored.
U.S. Pat. No. 5,573,679 discloses capacitive microphones that are fabricated using etch-release of sacrificial silicon by an isotropic dry etchant. The process allows the production of a microphone largely from chemical vapour deposition processes with flexibility in the materials selection. The dry etch chemistry does not require freeze-drying after release, and the etchant does not attack electrodes or metallized circuitry and so allows placement of the electrodes between the backplate and diaphragm dielectric layers. Diffusion barrier layers between the sacrificial and electrode layers protect both materials from interdiffusion during device fabrication. This process is especially suitable for forming a microphone comprising silicon nitride dielectric layers with aluminium electrodes.
U.S. Pat. No. 4,558,184 describes an electroacoustic transducer, such as a microphone, which may be integrated into a semiconductor chip, and a method of fabrication. The semiconductor is etched to produce a membrane having a sufficiently small thickness and an area sufficiently large that the membrane can vibrate at audio frequencies. Electrodes are provided in relation to the membrane so that an electrical output signal can de derived from the vibrations due to variable capacitance.
U.S. Pat. No. 4,533,795 describes an electroacoustic transducer, preferably in the form of a capacitive microphone, for incorporation into a semiconductor substrate. The vibrating element comprises a largely nontensioned diaphragm, such as an epitaxial layer formed on the semiconductor substrate, so as to greatly reduce its mechanical stiffness. The substrate is etched away in the desired area to define the diaphragm and form an acoustic cavity. A continuous array of microscopic holes is formed in the backplate to cut down the lateral flow of air in the gap between the capacitor electrodes. Narrow gaps made possible by the hole array allow low voltage diaphragm biasing.
Also known from the literature are Bergqvist et al. xe2x80x9cCapacitive microphone with a surface micromachined backplate using electroplating technologyxe2x80x9d, Journal of Microelectromechanical Systems Vol.3 No.2 (June 1994) pp.69-75 which describes a technology for surface micromachining of free-standing metal microstructures using metal electrodeposition on a sacrificial photoresist layer as applied to a condenser microphone. Murphy, P et al xe2x80x9cSubminiature silicon integrated electret condenser microphonexe2x80x9d 6th International Symposium on Elecrets (ISE 6) Proceedings describes a structure for a microphone which uses one silicon wafer to support a thin polyester diaphragm and a second to carry a Teflon electret. Subassemblies are diced from the wafer and bonded together to form complete microphones. Finally, Zou, Q. B. et al, xe2x80x9cDesign and fabrication of a novel integrated floating-electrode xe2x80x9cElectretxe2x80x9d microphone (FEEM)xe2x80x9d Proceedings of the 11th IEEE International Workshop on Micro Electro Mechanical Systems, January 1998. pp586-590. describes an electret microphone implemented by a single chip fabrication technique. The microphone uses a charged floating electrode surrounded by highly insulated materials as the xe2x80x9celectretxe2x80x9d to excite the electric field.
According to the present invention there is provided a semiconductor device comprising a microphone formed in an integrated manner and comprising a sensing electrode formed as part of an acoustic pressure sensing membrane, and a counter electrode in the form of a perforated rigid back-plate membrane, wherein said sensing electrode is connected to the gate of a sensing transistor.
In one embodiment the device may be operable in a constant bias mode. For example, the counter electrode may be set to a bias voltage and the potential of the sensing electrode and hence the gate potential of the sensing transistor may vary in accordance with the acoustic pressure, with the gate potential being biased in the conducting regime whereby variations in the gate voltage vary the output voltage of the sensing transistor.
Another possibility is that the potential of the sensing electrode may be fixed and an output current from the sensing electrode or the counter electrode and varying in response to acoustic pressure on the sensing membrane is taken as the output signal.
In another embodiment the device is operable in a constant charge mode by first charging the sensing electrode followed by setting the potential of the counter electrode to ground and biasing the sensing transistor to a conducting state. The charging of the sensing electrode may be by tunneling from a substrate, or may be by injection from the sensing transistor.
Preferably the sensing membrane is formed of a layer of an insulating material and a layer of a conducting material. The insulating material may comprise low stress silicon nitride, and the conducting material may comprise polysilicon.
Preferably the counter electrode is formed of a layer of a first conducting material and a layer of a second conducting material. In particular the second conducting material may be a relatively hard material and may be sandwiched between two layers of said first conducting material which is preferably a relatively soft material. For example the first material may be aluminium and the second material titanium.
Viewed from another broad aspect the present invention provides a method of forming an integrated semiconductor device including a microphone, comprising the steps of:
(a) depositing a first layer of an insulating material on a substrate to form a part of an acoustic sensing membrane,
(b) depositing a layer of conducting material on said layer of insulating material,
(c) depositing a second layer of an insulating material on said conducting material,
(d) depositing a sacrificial layer on said second layer of insulating material,
(e) depositing at least one rigid conducting material on said sacrificial layer to define a rigid counter electrode,
(f) forming a plurality of holes in said counter electrode,
(g) etching said substrate from a back side thereof to expose said membrane, and
(h) removing said sacrificial layer by etching through said holes to leave an airgap between said membrane and said counter electrode.
Preferably, before step (a), the substrate is subject to an isolation technique to separate on the substrate an area for the membrane, an area for a tunneling window, and areas for forming at least one sensing transistor and a substrate contact.
Preferably the conducting layer of the membrane also forms the gate electrode(s) of at least one sensing transistor.
Viewed from a still further broad aspect the present invention provides a method of forming a semiconductor device including a membrane, comprising the steps of:
(a) depositing a membrane forming material on a substrate,
(b) depositing a sacrificial layer of polysilicon on said membrane forming material,
(c) constructing a further semiconductor device structure on said substrate and over said sacrificial layer,
(d) removing said sacrificial layer, and
(e) etching a back side of said substrate to expose said membrane.
Viewed from a yet further broad aspect the present invention provides a method of forming a semiconductor device including a membrane, comprising the steps of:
(a) depositing a first membrane forming material on a substrate, said first membrane forming material being an insulating material,
(b) depositing a second membrane forming material on said first membrane forming material, said second membrane forming material being a conducting material and said second membrane forming material extending beyond said first membrane forming material so as to contact said substrate, whereby charges may be injected into said conducting material.
(c) depositing a sacrificial layer on said second membrane forming material,
(d) constructing a flier semiconductor device structure on said substrate and over said sacrificial layer,
(e) removing said sacrificial layer, and etching a back side of said substrate to expose said membrane.